1. Field of the Invention
The present invention relates to a color image forming apparatus of a so-called tandem system wherein a plurality of image forming stations individually forming toner images of different colors are arranged along a moving direction of a transfer medium. The invention further relates to a technique for correcting color registration offset.
2. Description of the Related Art
An apparatus set forth in Japanese Unexamined Patent Publication No.2004-109617, page 4, for example, has been known as this type of image forming apparatus. In this image forming apparatus, the image forming stations are arranged along the transfer medium, such as a transfer belt, on a per-color basis. The image forming station includes a charger, an image writing unit and a development unit which are disposed around a latent image carrier such as a photosensitive drum. Toner images formed by the individual image forming stations are mutually superimposed on the transfer medium, thereby forming a color image.
By the way, one of the serious problems encountered in the image forming apparatuses including a plurality of image forming stations is color registration offset. This problem results from mutual offset between/among transfer positions at which the individual toner images formed by the different image forming stations are transferred to the transfer medium. The color registration offset appears as a varied color tone. The following method is taken to solve this problem. Reference pattern images (hereinafter, referred to as “registration pattern images”) for detection of color registration offset are previously formed on the transfer medium. On the other hand, positional information related to the registration pattern images is acquired by detecting the registration pattern images by means of optical sensors. Based on the positional information thus acquired, the individual toner images are registered to each other (correction of color registration offset).